Well the purpose of a subjob was to either boost your main job's strength, compensate for its weakness, or use its strengths to change its role in a group. It wasn't meant to be used to do everything you want in one package but even though a damage dealing class class was always gonna be a DD, you could switch to THF sub and focus on enmity control/burst damage, switch to DNC sub to focus on TP fueled healing, or switch to WAR sub for pure damage and emergency provokes. Every MMO has roles that need to be filled. It's a group effort. Of course, if you had a job combination that doesn't help the group, why would they want you?
FFXIV is no different in that sense. The only difference is that instead of having a collection of -UNIQUE- jobs that can each fill several roles in a group by using different subjobs, you have a few DDs whose only difference is the weapon they use (save for a handful of class specific abilities), mages with no defined roles, and one standard tank. You aren't gonna see tanking archers, main healing gladiators, or melee DD mages (they can't even do that if they wanted to because everyone is bound to using one weapon). You would shun any one of those just like an FFXI player would shun a BLM/MNK.
Honestly, the armory system is made worse because of how unimaginative the classes are. Not to mention the awful battle system and physical levels contradicting the nature of the armory system. None of it works well together. People complain about the subjob system not giving them the freedom to do as they like but that isn't true, it's the community who doesn't let you do this and with good reason. This isn't a single player game where one character can deal physical damage, nuke, and heal at the same time and you being able to mix and match a bunch of random skills into one package will make you as unique as a monk subbing black mage in XI while wearing a set of CHR gear. It'll also make you just as useless.